Generally, on-wafer probing techniques are used to characterize MICs and devices. This requires the use of ground-signal-ground (G-S-G) probes based on a coplanar waveguide (CPW), as shown in FIG. 1, and a microwave precision network analyzer (PNA). For example, CPW 100 includes a strip conductor 105 with a ground plane 110 on each side of strip conductor 105. The CPW G-S-G probes are designed with a coaxial connector or a coax-to-waveguide transition at the input. A variant of this probe is also available as a signal-ground (S-G) or ground-signal (G-S) probe.
These probes, when connected via coaxial cables or waveguides to a PNA for RF measurements, allow a set-up to be calibrated using an impedance standard substrate (ISS). See, for example, FIG. 2, which shows a microstrip 200 with a strip conductor 205 on the top of microstrip 200 and a ground plane 210 beneath microstrip 200. FIG. 3 shows a slot line 300 having a signal coupled within a slot created by ground planes 305.
During the calibration process, software residing on the PNA analytically subtracts the losses associated with the probes and the cables/waveguides, and establishes a reference plane or calibration plane at probe tips. When physical contact is established between the calibrated probes and the device or circuit under test (DUT), the PNA measures and displays the intrinsic device or circuit scattering parameters (S-parameters) or characteristics.
At ambient temperatures, the above-mentioned procedure, which relies on good physical contact between the CPW G-S-G probes and the device or circuit pads, provides excellent results. However, in elevated temperature environments, such as in aircraft engine sensor development, the characteristics of the probe pads degrades and the physical contact with the probe becomes less reliable. In addition, the heat at elevated temperatures destroys the probe tips, which were designed to operate at room temperature. Replacing these probes would be expensive and procurement is time consuming.
Furthermore, in low temperature environments, such as in superconducting device development, the probes and the cables conduct the ambient heat to the cryogenically cooled device or circuit load placed inside a vacuum chamber. The excess heat manifests as an additional thermal load on the cryo-cooler, and thus, limits the lowest attainable temperature. Also, when the physical temperature of the device or circuit is lower, the associated noise figure is lower. When the noise figure is low, the sensitivity of the communications receiver is also higher. Moreover, CPW G-S-G probes tips are co-linear, and therefore, limited to device and circuit characterization on planar surfaces.
Thus, an alternative probe may be beneficial.